Biofilm Bacterial Density Calculator

∞ generated and posted on 2017.05.16 ∞

What I want to know, at least approximately, is the concentration of bacteria within biofilms, and with luck you can help me!

Please cite as:

Stephen T. Abedon
Biofilm Bacterial Density Calculator.

Click here for calculator or see immediately below for further explanation and discussion.

If you know the height of a biofilm (in μm) as well as its cell density per cm2, then it is easy to calculate the density of bacteria/ml. This is relevant since the likelihood of a phage adsorbing a bacterium is a function of bacterial density.

Recently I have done just this calculation based upon data from an oil-field pipeline. The relevant numbers 150 μm tall (or, if you prefer, thick) and 5 × 107 cells per cm2. The calculation and its solution are 3.3 × 109 cells/ml = (5 × 107 cells/108 μm2) × (1012 μm3/ml) × (1/150 μm).

In the calculator below I provide not just the estimated bacterial density for a biofilm but also the phage half life given that bacterial density. The smaller the half-life number, the faster the phages are expected to adsorb.

By default I provide the above numbers as well as a relatively low phage adsorption rate constant of 3 × 10-10 ml-1 min-1. Here the smaller the number, the slower the adsorption, but remember that the more negative the exponent, the smaller the number.

Feel free to modify these numbers in the calculator to get a sense of how changes in, e.g., per area cell density, biofilm height, or phage adsorption rate constant modifies the phage half life within a biofilm.

Below is the citation to this biofilm data:

Hamilton WA (1987) Biofilms: microbial interactions and metabolic activities, In: Fletcher, M., Gray, T.R.G., Jones, J.G. Editors, Ecology of Microbial Communities, Cambridge: Cambridge University Press, 361-385.

I'll supply further references and data as I become aware of them.

Note that it has been pointed out that we cannot assume that a biofilm consists of only a single type of bacterium (or phage-susceptibility type) and therefore that calculated densities will be of more than one type of bacterium. While that is a valid criticism at larger spatial scales (e.g., 1 mm), at smaller spatial scales (e.g., 10-100 μm) in fact we have a reasonable expectation that bacteria will be present as clonal microcolonies. So, yes, we are talking averages here when considering an entire biofilm, but at the same time the number of cells, to at least a first approximation, should be representative of what densities of susceptible bacteria a given phage might encounter while interacting with at least a portion of a biofilm.

Note also that with the calculator you can input titers and bacterial concentrations using "e" notation, as demonstrated above, on any of the lines except those indicated with an n. If you enter numbers in this way, however, make sure that the associated n lines are set to 0 (meaning 100 = 1), which is their default setting regardless.

If you need a refresher on scientific notation, there is always Wikipedia

= m
above is set to 1 if left blank or not number or ≤ 0
= n
above is set to 0 if left blank or not number or < 0

above is set to 100 if left blank or not number or ≤ 0

= m
above is set to 2.5 if left blank or not number or ≤ 0
= n
above is set to -9 if left blank or not number or > 0

Note k value of 2.5 × 10-9 ml-1 min-1 from Stent (1963)